The prior art contains several examples of duplex radio communications using Frequency Division Multiple Access (FDMA) in which different radio telephones each have a unique pair of frequencies for transmissions in the transmit and receive directions, for example, the U.S. AMPS cellular telephone system. The prior art also discloses duplex radio communication systems using Time Division Multiple Access (TDMA) in which each radio telephone has a unique time slot on a first shared frequency for communication in one direction, and a second unique time slot on a second shared frequency for communication in the other direction, for example, the European GSM digital system or the U.S. Digital Cellular standard IS-54. In these systems, the time slots in the respective directions are furthermore offset in time from each other so that the portable radiotelephones do not need to transmit and receive simultaneously. This eliminates the need for a transmit/receive duplexing filter which is required by a radiotelephone operating in a FDMA system. Instead, a so-called "time duplex" phone, as envisioned in the prior European Cellular System GSM, uses a simpler transmit/receive switch to couple the antenna alternately to the receiver or the transmitter.
In certain applications, neither TDMA nor FDMA provides an optimal solution. The TDMA system requires higher peak transmitter power to compensate for compressing the transmission into a time slot that is only a fraction of the total time, since it is the mean power that governs the range and quality of the communication. This is not an issue for a base station that in any case must have enough transmitter power to support all mobile stations, and the total power is the same for FDMA and TDMA solutions. It is simpler and cheaper for the TDMA base stations to have one high power transmitter and one antenna which can be timeshared between all base/mobile links using Time Division Multiplexing (TDM). However, it is often inconvenient for TDMA mobile stations to generate high peak power. On the other hand, it is inconvenient for FDMA mobiles to use antenna duplexing filters. Therefore, the present invention seeks to provide a method of using TDM on the base-to-mobile link (downlink) combined with FDMA on the mobile-to-base link (uplink), while avoiding the need for a duplexer.
The prior art discloses examples of mixed TDM/FDMA systems, such as the British Army's PTARMIGAN Single Channel Radio Access System (SCRA). The SCRA system is in fact a military radiotelephone system, and uses TDM on the downlink on a first frequency band while using FDMA on the uplink by allocating a separate frequency in a second frequency band to each mobile uplink. The SCRA system, however, requires either separate antennas for the uplink and downlink, respectively, or a duplexing filter to permit simultaneous transmission and reception through one antenna.
FIG. 1 illustrate the prior art transmission format described in the U.S. Digital Cellular standard IS-54. A base station transmits information continuously in frames of data which are 20 ms long. The data in question is composed of digitized speech information generated by a digital speech compression algorithm interspersed with synchronization, signalling and control symbols. Each 20 ms frame of data is divided into three time slots and each time slot contains information destined for one of three mobile stations. Thus, a particular mobile station only needs to turn on its receiver for one-third of the time since the data for the particular mobile station is confined to one of three time slots that make up the frame. In the reverse direction, the 20 ms frame is likewise divided into three time slots. Each mobile transmitter uses only one of the two time slots in which it is not receiving, which leaves the other third of the time which can be utilized to scan other base station frequencies to see if another base station is received more strongly. These signal strength measurements are reported over the uplink channel to the current base station, which makes a decision on whether to hand off communications with that mobile station to a stronger base station. Utilizing signal strength measurements performed by mobile stations in making handoff decisions is called "Mobile Assisted Handover" (MAHO).
In this prior art system, it can be seen that a mobile transmits for only one-third of the available time and therefore has to use three times the peak power that otherwise might have been sufficient if continuous transmission had been employed. If continuous transmission had been employed, all three mobile transmissions would be overlapping in time and would therefore have to be given different frequency channels as in the British Army's PTARMIGAN SCRA system. Furthermore, transmit/receive duplexing filters would be needed to allow simultaneous transmission and reception in the mobile station.